Nonablative state changeable data storage systems, for example, optical data storage systems, record information in a state changeable material that is switchable between at least two detectable states by the application of energy such as, for example, projected optical beam energy, electrical energy, or thermal energy, thereto.
State changeable data storage material is typically incorporated in a data storage device having a structure such that the data storage material is supported by a substrate and protected by encapsulants. In the case of optical data storage devices, the encapsulants include, for example, anti-ablation materials and layers, thermal insulation materials and layers, anti-reflection materials and layers, reflective layers, and chemical isolation layers. Moreover, various layers may perform more than one of these functions. For example, anti-reflection layers may also be anti-ablation layers and thermal insulating layers. The thicknesses of the layers, including the layer of state changeable data storage material, are engineered to minimize the energy necessary for effecting the state change and to optimize the high contrast ratio, high carrier to noise to ratio, and high stability of state changeable data storage materials.
The state changeable material is a material capable of being switched from one detectable state to another detectable state or states by the application of, for example, projected beam energy, electrical energy, or thermal energy thereto. The detectable states of state changeable materials may differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties, including indices of refraction and reflectivity, or combinations of one or more of the foregoing. The state of the state changeable material is detectable by properties such as, for example, the electrical conductivity, electrical resistivity, optical transmissivity, optical absorption, optical refraction, optical reflectivity, or combinations thereof. That is, the magnitude of the detectable property will vary in a predictable manner as the state changeable material changes state.
Formation of the data storage device includes deposition of the individual layers by, for example, evaporative deposition, chemical vapor deposition, and/or plasma deposition. As used herein plasma deposition includes sputtering, glow discharge, and plasma assisted chemical vapor deposition.
Tellurium based materials have been utilized as state changeable materials for data storage where the state change is a structural change evidenced by a change in a physical property such as reflectivity. This effect is described, for example, in J. Feinleib, J. deNeufville, S. C. Moss, and S. R. Ovshinsky, "Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors," Appl. Phys. Lett., Vol. 18(6), pages 254-257 (Mar. 15, 1971), and in U.S. Pat. No. 3,530,441 to S. R. Ovshinsky for Method and Apparatus For Storing And Retrieving Of Information.
Tellurium based state changeable materials, in general, are single or multi-phased systems and: (1) the ordering phenomena include a nucleation and growth process (including both or either homogeneous and heterogeneous nucleations) to convert a system of disordered materials to a system of ordered and disordered materials; and (2) the vitrification phenomena include melting and rapid quenching of the phase changeable material to transform a system of disordered and ordered materials to a system of largely disordered materials. The above phase changes and separations occur over relatively small distances, with intimate interlocking of the phases and gross structural discrimination, and are highly sensitive to local variations in stoichiometry.
In chalcogenide type memory materials, the measures of performance include (1) the contrast ratio, that is, the difference in reflectivities of the states divided by the sums of the reflectivities of the states, and (2) the carrier to noise ratios of both (a) the "written" and (b) the "erased" states. The failure mode of the memory material is evidenced by the deterioration in the measures of performance with respect to the number of cycles. That is, failure may be evidenced by for example (1) a reduction in contrast ratio with increasing cycles, or by (2) a reduction in the written carrier to noise ratio or an increase in the erased carrier to noise ratio.
If the memory material is to be used as a "write once" medium, it is highly desirable that it have a relatively high glass transition temperature since this gives it good thermal stability over the life of the recording medium. Since, in many cases, it is customary to write or record onto a medium which is already in an amorphous first state by switching it to crystalline second state, the integrity of the recording depends on the ability of the amorphous second state to resist spontaneous or accidental crystallization into the written state. The higher the glass transition temperature of the recording medium is, the more likely the medium will resist undesirable crystallization when in the amorphous state, particularly when the recorded region consists of all alternating regions of recorded crystalline spots and unaltered amorphous species between. Of course, the crystallization temperature must not be so high or that recording sensitivity suffers.
In addition to write once materials, optical data storage systems may also employ erasable material which may be recorded upon, erased, rerecorded upon, re-erased, etc. In this case, the phase changeable material must reversibly be able to change from the amorphous to the crystalline state, and back again, repeatedly. For erasable materials, additional considerations are important. Most significantly, through repeated cycle life (repeated vitrification and crystallization), prior art materials have shown a tendency to degrade over time. It is thought that important factors contributing to this degradation over cycle life are the tendency for inhomogeneities and inclusions to appear in the material. That is, after the material crystallizes and recrystallizes, regions of variable structure and composition may appear therein which were not present in the original material. Due to the thermal history of the material, the various components of which it is composed may selectively migrate, chemically bond, or substitute for other components in the crystal lattice. Moreover, some of the material may not properly vitrify, thus creating undesirable crystalline or noncrystalline inclusions. All of these factors can result in degraded performance.
These problems become particularly acute in typical tellurium based materials, such as tellurium-antimony-germanium systems because, depending upon the atomic percentages of each individual component within the material, these TAG systems can form a multitude of crystalline and amorphous phases. Thus, even though the as deposited material has a nominal composition of Te.sub.x Ge.sub.y Sb.sub.z, the material, in its crystalline state, may comprise a plurality of crystalline phases of varying and unpredictable composition. That is, one crystalline phase may have proportionately much more tellurium, another much more germanium, etc. Because of this noncongruency of composition between the amorphous and crystalline states, such materials are prone to degrade over cycle life due to the reasons explained above.
Furthermore, due to the multitude of crystalline phases, switching to the crystalline state includes atomic diffusion and is therefore relatively slow. Thus, switching speed is undesirably slow, and selecting a new composition to improve switching speed may compromise and the thermal stability of the material.
The desirability of congruency in composition between the crystalline and amorphous states has been described with respect to a binary optical memory material in U.S. Pat. Nos. 4,876,667 and 4,924,436, both assigned to assignee of the present invention. Both referenced patents concern themselves with binary chalcogenide data storage material, namely antimony telluride/antimony selenide compositions. As described in the referenced patents, the telluride and the selenide are essentially compositionally congruent between the amorphous and crystalline states. This is so because the telluride/selenide compositions are substantially miscible in substantially all proportions. That is, each crystal has approximately the nominal composition of the as deposited material. This is so largely because, due to their atomic structure and valence characteristics, the tellurides and selenides are capable of substituting for each other in the crystal lattice over a wide range of compositions. However, this is known not to be the case for TeGeSb systems.
The desirability of maintaining compositional congruency while depositing the layer of memory material in tellurium-based memory materials has been recognized. See, for example, U.S. Pat. No. 4,621,032, assigned to the assignee of the present invention. The reference discloses a method of depositing the layer of memory material by a congruent sublimation process wherein the sources or precursor materials are selected so as to maintain consistency in the deposited materials. However, this patent also teaches that there can be phase separation upon crystallization of the amorphous material into a crystalline and amorphous second phase, which phase separation necessarily entails the undesirable consequences noted above.